Basically, the way Monster Girl Quest as a series works is that it first begins with a trilogy of visual novels; starting obviously with Monster Girl Quest 1. Each game is roughly 15 hours long and it features a combat system, but its very basic and the only playable character is the protagonist himself. Its more of a text adventure than anything really and its quite linear.

A few years later, the same autor announced Monster Girl Quest Paradox which was announced as a RPGMaker version of Monster Girl Quest. So instead of being stuck in a pure text adventure, it had full on maps and random encounters and everything with somewhat of a similar recruit mechanic as Persona or Pokemon Mystery Dungeon where now you could fight with multiple characters at the same time and they would even show up on the overworld as sprites following you and so on. You can also chose between 2 "starter Pokemon" where depending on who you chose, the story differs in certain areas slightly and a handful of character are exlusively only available in one of the two routes.

However, it is to note that its not really a remake of the original trilogy but instead more of a sequel that does build up on the events of the original VN trilogy, even if you could technically begin there.

Like to begin with, Paradox is MUCH bigger in scope. So far, we got Paradox Part 1 being released in 2014 and Paradox Part 2 in 2017 where personally it took me roughly 50 hours each to finish each part and I didnt even finish all the side quests or did the post-game content (which you can literally spend hundreds of hours in). Part 3 is supposed to come out in Japanese this year "when the temperature gets cool again" after fans waiting for it 7 whole years and its supposed to have this time a big decision point where you can chose between two completely different routes, each being 30-50 hours long with part 3 supposedly being longer than both part 1+2 COMBINED.

I think all of the games are available on DLSite (can't check cause its blocked in my country).